WAITPID(3C) Standard C Library Functions WAITPID(3C)

NAME

waitpid - wait for child process to change state

SYNOPSIS

#include <sys/types.h>
#include <sys/wait.h>

pid_t waitpid(pid_t pid, int *stat_loc, int options);

DESCRIPTION

The waitpid() function will suspend execution of the calling thread until
status information for one of its terminated child processes is
available, or until delivery of a signal whose action is either to
execute a signal-catching function or to terminate the process. If more
than one thread is suspended in waitpid(), wait(3C), or waitid(2)
awaiting termination of the same process, exactly one thread will return
the process status at the time of the target process termination. If
status information is available prior to the call to waitpid(), return
will be immediate.


The pid argument specifies a set of child processes for which status is
requested, as follows:

o If pid is less than (pid_t)-1, status is requested for any
child process whose process group ID is equal to the absolute
value of pid.

o If pid is equal to (pid_t)-1, status is requested for any
child process.

o If pid is equal to (pid_t)0 status is requested for any child
process whose process group ID is equal to that of the
calling process.

o If pid is greater than (pid_t)0, it specifies the process ID
of the child process for which status is requested.


One instance of a SIGCHLD signal is queued for each child process whose
status has changed. If waitpid() returns because the status of a child
process is available, and WNOWAIT was not specified in options, any
pending SIGCHLD signal associated with the process ID of that child
process is discarded. Any other pending SIGCHLD signals remain pending.


If the calling process has SA_NOCLDWAIT set or has SIGCHLD set to SIG_IGN
and the process has no unwaited children that were transformed into
zombie processes, it will block until all of its children terminate, and
waitpid() will fail and set errno to ECHILD.


If waitpid() returns because the status of a child process is available,
then that status may be evaluated with the macros defined by
wait.h(3HEAD) If the calling process had specified a non-zero value of
stat_loc, the status of the child process will be stored in the location
pointed to by stat_loc.


The options argument is constructed from the bitwise-inclusive OR of zero
or more of the following flags, defined in the header <sys/wait.h>:

WCONTINUED
The status of any continued child process specified by pid,
whose status has not been reported since it continued, is
also reported to the calling process.


WNOHANG
The waitpid() function will not suspend execution of the
calling process if status is not immediately available for
one of the child processes specified by pid.


WNOWAIT
Keep the process whose status is returned in stat_loc in a
waitable state. The process may be waited for again with
identical results.


WUNTRACED
The status of any child processes specified by pid that are
stopped, and whose status has not yet been reported since
they stopped, is also reported to the calling process.
WSTOPPED is a synonym for WUNTRACED.

RETURN VALUES

If waitpid() returns because the status of a child process is available,
it returns a value equal to the process ID of the child process for which
status is reported. If waitpid() returns due to the delivery of a signal
to the calling process, -1 is returned and errno is set to EINTR. If
waitpid() was invoked with WNOHANG set in options, it has at least one
child process specified by pid for which status is not available, and
status is not available for any process specified by pid, then 0 is
returned. Otherwise, -1 is returned and errno is set to indicate the
error.

ERRORS

The waitpid() function will fail if:

ECHILD
The process or process group specified by pid does not exist or
is not a child of the calling process or can never be in the
states specified by options.


EINTR
The waitpid() function was interrupted due to the receipt of a
signal sent by the calling process.


EINVAL
An invalid value was specified for options.

USAGE

With options equal to 0 and pid equal to (pid_t)-1, waitpid() is
identical to wait(3C). The waitpid() function is implemented as a call to
the more general waitid(2) function.

ATTRIBUTES

See attributes(7) for descriptions of the following attributes:


+--------------------+-------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-------------------+
|Interface Stability | Committed |
+--------------------+-------------------+
|MT-Level | Async-Signal-Safe |
+--------------------+-------------------+
|Standard | See standards(7). |
+--------------------+-------------------+

SEE ALSO

Intro(2), exec(2), exit(2), fork(2), pause(2), sigaction(2), ptrace(3C),
signal(3C), wait(3C), siginfo.h(3HEAD), wait.h(3HEAD), attributes(7),
standards(7)

December 7, 2007 WAITPID(3C)